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Full text of "The Cambridge Natural History"

FINS OF WHALES                                    41

on p. 196. The only mammal which appears to have the proper
five bones in the distal row of the carpus corresponding to the
five metacarpals is Hyperoodon, where this state of affairs at least
occasionally occurs. The final bone of that series, the unciform,
seems to represent two bones fused. Very often the carpus
is reduced by the fusion of certain of the carpal bones; thus
among the Garnivora it is usual for the scaphoid and the lunar
to be fused. It is interestingly significant that these bones retain
their distinctness in the ancestral Creodonts. In many Ungulates
the trapezium vanishes. The reduction of the toes in fact implies
a reduction of the separate elements of the carpus.
As to the digits of the mammalian hand, the greatest number
is five, the various supplementary bonelets known as prepollex
and postimnimus being, it is now generally held, merely supple-
mentary ossifications not representing the rudiments of pre-existing
fingers. They may, however, bear claws.1 The number of
phalanges which follow upon the metacarpals is almost constantly
three in the mammals, excepting for the thumb, which has only two.
This is highly characteristic of the group as opposed to reptiles
and birds, and the increase in the number of these bones in the
WTiales and to a very faint degree in the Sirenia is a special re-
duplication, which will be mentioned when those animals are
treated of.
The Pelvic Girdle.—The pelvic girdle or hip girdle is the
combined set of bones which are attached on the one hand to
the sacrum and on the other articulate with the hind-limb.
Four distinct elements are to be recognised in each " os inno-
minatum/' the name given to the conjoined bones of each half of
the entire pelvis. These are :—the ilium, which articulates with
the sacrum ; the ischium, which is posterior ; the pubis, which is
anterior; and finally, a small element, the cotyloid, which lies
within the acetabular cavity where the femur articulates. The
epipubes of the Monotreme and the Marsupial are dealt with
elsewhere (see p. 116) as they are peculiar to those groups.
Professor Huxley pointed out many years since that while
the Eutherian Mammalia differ from the reptiles in the fact that
the axis of the ilium lies at a less angle with that of the sacrum,
1 Horny matter is apt to be formed upon extremities ; instances which are well
known are the "claws" upon the tail of the Lion and Leopard and the Kangaroo
Onychogale. For an account of the first see Proc. Zool. Son, 1832, p. 146.